The invention relates to a method for leak testing, in particular for detecting a leak gas, for example, in a vacuum device, using a flushing gas method. The invention also relates to a leak detection apparatus for carrying out the method.
In research and industrial technology, many uses are made of vacuum devices. Depending on the particular operating conditions, certain requirements are set for keeping up the vacuum in the vacuum device. The ingress of gases into the vacuum device from the surroundings due to leaks has to be prevented as far as possible. For example, in vacuum devices for plasma research, it is a requirement that all components have a leak rate at all operating temperatures of less than 10−10 Pa m3/s. Searching for leaks represents a challenge also in the context of other technical fields, for example, in chemical plants or combustion plants in the prevention, for example, of the undesirable escape of a gas into the surroundings.
Known methods for leak detection are, in particular, the vacuum method, the positive pressure method and the sniff testing method. A disadvantage of the vacuum method is that a vacuum chamber adapted to the geometry of the component to be tested must be available. With complex vacuum equipment, this results in an extremely high testing cost. A disadvantage of the positive pressure method and of the sniff testing method is the substantially time-consuming nature of these methods. In addition, the detection limits of these methods have previously been in the range of 10−5 to 10−7 Pa m3/s and therefore significantly above the aforementioned required leak rate. A further disadvantage of conventional sniff testing lies therein that detection of leaks from components that are cooled to the temperature of liquid nitrogen has not previously been possible with such methods.
The locating of leaks with the sniff testing method or the positive pressure method is difficult or even impossible where equipment is of complex construction. Therefore, a flushing gas method has been developed for leak detection. An example of the use of the flushing gas method is described in DE 103 06 245. A local gas space in which a flushing gas is situated is provided round a component to be tested. The local gas space in which the flushing gas is situated is tested for traces of a leak gas that has been placed in the interior of the component. The search for the leak gas is carried out, for example, as it is with sniff testing, using a mass spectrometer. In the case of the method described in DE 103 06 245, ambient air is drawn into the housing as the flushing gas. However, due to the inadequate purity of the ambient air, this method is unsuitable for leak detection in the case of the extremely low detection limits required, for example, with vacuum equipments.
From the presentation at the DGZfP annual conference entitled “Non-destructive material testing”, 21-23 May 2001 in Berlin (report vol. 75) and the DIN ISO 15848-1 standard which is based thereon, a variant of the flushing gas method is known wherein it is not ambient air but rather a flushing gas from a flushing gas source that is used for the flushing. With this method, which is illustrated schematically in FIG. 17, a leak detection apparatus 100′ comprises a flushing device 10′ on a component 201′ to be tested, a gas feed device 20′ with which the flushing gas can be introduced into the flushing device 10′, and a detector device 30′ for detecting leak gas in the flushing gas which emerges from the flushing device 10′. The gas feed device 20′ comprises a flushing gas source 23′, a pressure regulator 24′, a flow control system 25′ and a flow measurement device 26′. By means of the gas feed device 20′, a specific system flow of the flushing gas which is introduced into the flushing device 10′ (see arrow) is generated under the action of an excess pressure set and monitored with the components 24′ to 26′.
In order to be able to carry out a precise and reproducible measurement with the detector 31′ of the detector device 30′, when the system flow is adjusted, the following requirements must be met. A constant positive pressure must be maintained in the flushing device 10′. The system flow must be adjusted so as to compensate for the pressure loss arising from the outflow of flushing gas to the detector device 30′. One problem consists therein that this pressure loss is not constant in practice. As a result of temperature changes, shape changes of the flushing device 10′ which is formed e.g. as a flexible covering, or the piercing of a probe of the detector device 30′ into the flushing device 10′, pressure variations which directly impair the measuring result from the detector 31′ can occur.
Furthermore, with the conventional methods, in order to avoid reverse diffusion of ambient air via the detector device 30′ into the flushing device 10′, a relatively large system flow is needed. This is disadvantageous with large pressure variations in the flushing device 10′ and is therefore associated with a worsening in the detection limit for leak gas detection.
The conventional method as illustrated in FIG. 17 therefore suffers from the disadvantage that the stated requirements placed on the leak detection can only be fulfilled with difficulty and adjustment of the system flow of the flushing gas is only possible to a limited degree of accuracy.
It is an objective of the invention to provide an improved method for leak testing by means of which the disadvantages of the conventional methods are overcome and which is characterized, in particular, by more stable adjustment of a flushing gas flow, an improved detection limit for the leak gas detection, and increased testing speed. Furthermore, the method should be capable of being carried out with a high degree of flexibility under various operating conditions and of being automated. It is a further objective of the invention to provide an improved leak detection device, with which the disadvantages of the conventional leak detection devices can be overcome and which enables leak detection with an improved detection limit and at increased speed.
These objectives are achieved with methods and devices of the invention.